Montane Featherlite Pertex over trousers.
Their made of fairly thin material, and i recon getting them stuck in a few brambles would do them no favours, but their ultra lightweight, and you can hide them inside the palm of your hand.
Their incredibly windproof, and ive never seen anything dry so quickly.
Their not cheap, but they have full ankle zips, are beautifully made in the lightest pertex available, and boy are they windproof.
Their only downside, is that their not the slightest bit waterproof, or showerproof, but from wet to being dry, incredibly fast drying.
Almost a miracle of nature, their dry in under four minutes in sunny windy conditions, and they weigh next to nothing.
They pack so small, that you could put them in your jacket pocket, and forget their there.