A:Wearing of trousers/pants by females has been praised in the Hadeeth. A Hadeeth mentions that the pants provides more concealment and is a protection for women. While expressing his happiness, Rasulullah (Sallallaahu Alayhi wa Sallam) also made Du’aa of forgiveness thrice for those females among the Ummah who wear trousers. (Al Aadaab Bayhaqi V2 P194).
If the abaaya is thick, not see-through and no portion is exposed during the Salaah because of the movements, the Salaah will be valid but modesty demands that she wear a pants.
Allah Ta’aala knows best.
Moulana Yusuf Laher