The Minchas Yitzchak 2:108, Shevet HaLevi YD:63, Tzitz Eliezer 11:2, Sha’arei Shalom on Kitzur Shulchan Arukh, Mevaser Shalom (notes 3:2) say that pants are Beged Ishm – “A man’s clothing shall not be worn by a woman” (Devorim 22:5). Lo Yilbash.
Even those that don’t refer to Beged Ish, prohibit it due to tznius. (i.e. Yabia Omer 6
Siman 14, Mekor Chaim, Rav Ovadia Yosef, Yaskil Avdi)
Halichos Bas Yisrael quotes R’ Scheinberg and R’ Elyashiv as well as prohibiting it. iIt is clear that all thse poskim hold that wearing pants is prohibited.
(Pesachim 3a states that it is not proper for women to straddle a horse or donkey because of immodesty.)