  We demonstrate that flow of longitudinal unpolarized current through a
ballistic two-dimensional electron gas with Rashba spin-orbit coupling will
induce nonequilibrium spin accumulation which has opposite sign for the two
lateral edges and it is, therefore, the principal observable signature of the
spin Hall effect in two-probe semiconductor nanostructures. The magnitude of
its out-of-plane component is gradually diminished by static disorder, while it
can be enhanced by an in-plane transverse magnetic field. Moreover, our
prediction of the longitudinal component of the spin Hall accumulation, which
is insensitive to the reversal of the bias voltage, offers a smoking gun to
differentiate experimentally between the extrinsic, intrinsic, and mesoscopic
spin Hall mechanisms.
