  A Datta-Das spin field effect transistor is built of a one-dimensional weak
link, with Rashba spin orbit interactions (SOI), which connects two magnetized
reservoirs. The particle and spin currents between the two reservoirs are
calculated to lowest order in the tunneling through the weak link and in the
wide-band approximation, with emphasis on their dependence on the origins of
the `bare' magnetizations in the reservoirs. The SOI is found to generate
magnetization components in each reservoir, which rotate in the plane of the
electric field (generating the SOI) and the weak link, only if the `bare'
magnetization of the other reservoir has a non-zero component in that plane.
The SOI affects the charge current only if both reservoirs are polarized. The
charge current is conserved, but the transverse rotating magnetization current
is not conserved since the SOI in the weak link generates extra spin
polarizations which are injected into the reservoirs.
