  The concept of ratchets, driven asymmetric periodic structures giving rise to
directed particle flow, has recently been generalized to a quantum ratchet
mechanism for spin currents mediated through spin-orbit interaction. Here we
consider such systems in the coherent mesoscopic regime and generalize the
proposal of a minimal spin ratchet model based on a non-interacting clean
quantum wire with two transverse channels by including disorder and by
self-consistently treating the charge redistribution in the nonlinear
(adiabatic) ac-driving regime. Our Keldysh-Green function based quantum
transport simulations show that the spin ratchet mechanism is robust and
prevails for disordered, though non-diffusive, mesoscopic structures. Extending
the two-channel to the multi-channel case does not increase the net ratchet
spin current efficiency but, remarkably, yields a dc spin transmission
increasing linearly with channel number.
