  We investigate intersubband mixing effects in multichannel quantum wires in
the presence of Rashba spin-orbit coupling and attached to two terminals. When
the contacts are ferromagnetic and their magnetization direction is
perpendicular to the Rashba field, the spin-transistor current is expected to
depend in a oscillatory way on the Rashba coupling strength due to spin
coherent oscillations of the travelling electrons. Nevertheless, we find that
the presence of many propagating modes strongly influences the spin precession
effect, leading to (i) a quenching of the oscillations and (ii) strongly
irregular curves for high values of the Rashba coupling. We also observe that
in the case of leads' magnetization parallel to the Rashba field, the
conductance departs from a uniform value as the Rashba strength increases. We
also discuss the Rashba interaction induced current polarization effects when
the contacts are not magnetic and investigate how this mechanism is affected by
the presence of several propagating channels.
