  We report the observation of an anomalous conductance plateau near G = 0.5 G0
(G0 = 2e2/h) in asymmetrically biased AlGaAs/GaAs quantum point contacts
(QPCs), with in-plane side gates in the presence of lateral spin-orbit
coupling. This is a signature of spin polarization in the narrow portion of the
QPC. The appearance and evolution of the conductance anomaly has been studied
at T=4.2K as a function of the potential asymmetry between the side gates. The
observation of spontaneous spin polarization in a side-gated GaAs QPC could
eventually lead to the realization of an all-electric spin-valve at tens of
degrees Kelvin.
