  Controlling the flow of electrons by manipulation of their spin is a key to
the development of spin-based electronics. While recent demonstrations of
electrical-gate control in spin-transistor configurations show great promise,
operation at room temperature remains elusive. Further progress requires a
deeper understanding of the propagation of spin polarization, particularly in
the high mobility semiconductors used for devices. Here we report the
application of Doppler velocimetry to resolve the motion of spin-polarized
electrons in GaAs quantum wells driven by a drifting Fermi sea. We find that
the spin mobility tracks the high electron mobility precisely as a function of
T. However, we also observe that the coherent precession of spins driven by
spin-orbit interaction, which is essential for the operation of a broad class
of spin logic devices, breaks down at temperatures above 150 K for reasons that
are not understood theoretically.
