Lightweight down pants. These are custom made to order. You can choose the shell material based on weight, durability or water repelancy and breathability. Full-length separating zippers and baffled construction.
These pants are lightweight and highly packable, yet they insulate as well as I could ever want, even in -20F conditions. You can specify that you want a zip fly (which is not standard), which is important for access the tie in point on your harness if you wear the pants over the harness, which is typical. Specify an inverted zipper for convenience with a harness. I ordered mine with the 800 overfill option, which increases the weight to warmth and packability. For $8 bucks more, you can't go wrong.\
Feathered friends is one of those mystery companies. They custom make gear in Seattle that is better than the mass-produced stuff made in China but costs the same. Go figure.