  Electronic structure calculations for layered zincblende semiconductors are
described within a restricted basis formalism which naturally and
non-perturbatively accomodates both crystalline inversion asymmetry and cubic
anisotropy. These calculations are applied to calculate the electron spin
decoherence times $T_1$ and $T_2$ due to precessional decoherence in quantum
wells. Distinctly different dependences of spin coherence times on mobility,
quantization energy, and temperature are found from perturbative calculations.
Quantitative agreement between these calculations and experiments is found for
GaAs/AlGaAs, InGaAs/InP, and GaSb/AlSb $(001)$-grown quantum wells. The
electron spin coherence times for CdZnSe/ZnSe II-VI quantum wells are
calculated, and calculations of InGaAs/GaAs quantum wells appropiate for
comparison with spin-LED structures are also presented.
