  We investigate quantum coherence of electron spin transported through a
semiconductor spintronic device, where spins are envisaged to be controlled by
electrical means via spin-orbit interactions. To quantify the degree of spin
coherence, which can be diminished by an intrinsic mechanism where spin and
orbital degrees of freedom become entangled in the course of transport
involving spin-orbit interaction and scattering, we study the decay of the
off-diagonal elements of the spin density matrix extracted directly from the
Landauer transmission matrix of quantum transport. This technique is applied to
understand how to preserve quantum interference effects of fragile
superpositions of spin states in ballistic and non-ballistic multichannel
semiconductor spintronic devices.
