  Spin drag measurements were performed in a two-dimensional electron system
set close to the crossed spin helix regime and coupled by strong intersubband
scattering. In a sample with uncommon combination of long spin lifetime and
high charge mobility, the drift transport allows us to determine the spin-orbit
field and the spin mobility anisotropies. We used a random walk model to
describe the system dynamics and found excellent agreement for the Rashba and
Dresselhaus couplings. The proposed two-subband system displays a large tuning
lever arm for the Rashba constant with gate voltage, which provides a new path
towards a spin transistor. Furthermore, the data shows large spin mobility
controlled by the spin-orbit constants setting the field along the direction
perpendicular to the drift velocity. This work directly reveals the resistance
experienced in the transport of a spin-polarized packet as a function of the
strength of anisotropic spin-orbit fields.
