  We studied the spin-dependent quantum transport properties using a simple
modelling of a Datta-Das spin transistor. We refine previous results by
accounting the propagation medium changes of opacity felt by itinerant
electrons, when the gate-voltage is switched on and modelling them via the
transversal energy levels mismatch. Monitoring the topological-dependent
conductance, we are able to identify the device operating points. If the
incoming electrons energy approaches the biased-induced barriers height, the
spin-resolved conductance oscillations become significant. In a zero
temperature picture, our computations of the spin-dependent conductance as
function of the electric field at the region below the gate electrode suggest
the feasibility of the modeled device. Although we demonstrate that phase time
may not be spin-resolved, our simulation allows us to evaluate the time that
takes an electron to experience a spin-flip process, resulting in an order of
magnitude lower than typical values of the spin relaxation times.
