  We consider Rashba spin-orbit effects on spin transport driven by an electric
field in semiconductor quantum wells. We derive spin diffusion equations that
are valid when the mean free path and the Rashba spin-orbit interaction vary on
length scales larger than the mean free path in the weak spin-orbit coupling
limit. From these general diffusion equations, we derive boundary conditions
between regions of different spin-orbit couplings. We show that spin injection
is feasible when the electric field is perpendicular to the boundary between
two regions. When the electric field is parallel to the boundary, spin
injection only occurs when the mean free path changes within the boundary, in
agreement with the recent work by Tserkovnyak et al. [cond-mat/0610190].
