As others have noted, metal will often get you flagged. For this reason, if you need to wear pants with some type of suspension, you should definitely opt for something like sweat pants with a cloth drawstring. Of course, there are many belts which have no metal parts, but if your pants have a zipper or button or any metal fasteners, then having a non-metal belt will likely be insufficient to avoid a pat-down. I have a pair of outdoor hiking pants with removable lower legs and a nylon belt with a plastic buckle. All the zippers are very small/lightweight; but because it has zippers near the knee (which I’m sure shows up as anomalous density in the pant legs), I get a pat-down when wearing them.
As others have said, having a scrunched-up tissue in your pocket is enough to trigger a pat-down. So when they say “empty your pockets”, make sure there is nothing more than tiny pieces of lint left in them.
I like to wear shorts myself (athletic wear, not Dockers), but the plane can get cold depending on time of day/airline/duration/alignment of the planets/etc. So baggy sweat pants with no metal is likely the safest bet. FWIW, I think tight-fitting pants would reduce your chances of getting a pat-down, because the fibers would be stretched, and therefore more transparent to scanning. However, as long as your pants are not unusually thick, I don’t think it is really necessary to resort to such measures.
If you want to maximize your clothing transparency to millimeter wave detectors, which is the same as saying “look as naked as possible at millimeter wave frequencies”, then you can tweak things even further with your material choice, courtesy of the US Army. Take note of Figure 5, on page 6. Transparency, ranked from best to worst is:
Of course, I’m hand-waving a bit here, as each material overlaps the other on some data points (frequencies), and I have no idea what wavelength is used by actual scanners, or whether it’s common or varies a lot. The ultimate source for this data comes from another study (again paid partly by the US Army), which has this helpful summary in the abstract:
All samples were usefully transparent at millimeter-wave frequencies
(up to 300GHz) based on a 3dB criterion, but became progressively
opaque at higher frequencies in a highly sample-dependent manner. This
is explained by the samples becoming “optically dense” in the THz
region, so that the transmission becomes exponentially dependent on
sample thickness. The attenuation in the IR region is very high
(⩾25dB) except in two samples (rayon and nylon), whose exceptional
transparency (e.g., −12dB in nylon) is attributed to pores intrinsic
to the material.
So, if you really wanna get your “airport security naked” on, then go for the thinnest nylon or rayon pants sans metal that you can find. I’d recommend pantyhose, but that will probably get you flagged for other reasons. 😉 Note that material thickness also matters, so if you go the sweat pants route, thick, heavy sweat pants will work against your goal. Silk or nylon pajama pants sounds like the winner.