  Resonant behavior involving spin-orbit entangled states occurs for spin
transport along a narrow channel defined in a two-dimensional electron gas,
including an apparent rapid relaxation of the spin polarization for special
values of the channel width and applied magnetic field (so-called ballistic
spin resonance). A fully quantum mechanical theory for transport through
multiple subbands of the one-dimensional system provides the dependence of the
spin transport on the applied magnetic field and channel width, including a
resonant depolarization of spins when the Zeeman energy matches the subband
energy splittings and a spin texture transverse to the magnetic field. The
resonance phenomenon is robust to disorder.
