In reply to Carless:
I was just thinking about trousers, yesterday, on my ascent on our local small hill. Trousers pose a bit of a problem, don't they, because, where conditions to deteriorate in a way that they became wet and one were to need to trade in some breathability for more waterproofness, one thing that most sane people really, really don't want to be doing is faffing about, taking their ski-boots off and changing their pants!
So ski-touring trousers need to handle warm-and-dry, cold-and-dry *and*, to at least some extent, moist.
I don't carry rain-trousers when skiing, today, because, when I did in the past, I never used them. I invariably just hurried up and got down or to shelter instead of getting wetter, changing. Were I to get into trouble in the wet, I've got a bivvy sack in my pack... although whether that will help, who knows?
I go for any old soft-shell that seems like it will handle a bit of abrasion and fits over the top of my boots. After that, a couple of features are key.
For me, one absolutely mandatory feature is the ability to zip open the legs, for sunny ascents. It doesn't matter what your trousers are made of if they're sweated through and the weather turns cold or windy for the summit and descent! Plus, if you sweat too much, your socks might lose their insulating properties -- cold, frozen toes can't make nice turns so the ski down will be miserable.
Another must-have feature is a zippable waist-pocket for your beacon. Sure, wearing it on your chest *is* possible but that requires you to be wearing at least two top layers, if you follow recomendations to the letter and don't wear it against your skin or outside your clothes. Here in the Alps, sure, we have cold tours but I find I'm nearly always stripped to a T-shirt for the ascent on the vast majority of them.
Any old soft-shell that ticks these boxes is fine, by me.