  The appearance and evolution of an anomalous conductance plateau at 0.4 (in
units of 2e2/h) in an In0.52Al0.48As/InAs quantum point contact (QPC), in the
presence of lateral spin-orbit coupling, has been studied at T=4.2K as a
function of the potential asymmetry between the in-plane gates of the QPC. The
anomalous plateau, a signature of spin polarization in the channel, appears
only over an intermediate range (around 3 V) of bias asymmetry. It is quite
robust, being observed over a maximum range of nearly 1V of the sweep voltage
common to the two in-plane gates. Our conductance measurements show evidence of
surface roughness scattering from the side walls of the QPC. We show that a
strong perpendicular magnetic field leads to magnetic confinement in the
channel which reduces the importance of scattering from the side walls and
favors the onset of near ballistic transport through the QPC.
