  We study the non-equilibrium dynamics of electron transmission from a
straight waveguide to a helix with spin-orbit coupling. Transmission is found
to be spin-selective and can lead to large spin polarizations of the itinerant
electrons. The degree of spin selectivity depends on the width of the interface
region, and no polarization is found for single-point couplings. We show that
this is due to momentum conservation conditions arising from extended
interfaces. We therefore identify interface structure and conservation of
momentum as crucial ingredients for chiral-induced spin selectivity, and
confirm that this mechanism is robust against static disorder.
