  A theory of spin injection across a ballistic
ferromagnet-semiconductor-ferromagnet junction is developed for the Boltzmann
regime. Spin injection coefficient $\gamma$ is suppressed by the Sharvin
resistance of the semiconductor $r_N^*=(h/e^2)(\pi^2/S_N)$, where $S_N$ is the
Fermi-surface cross-section. It competes with the diffusion resistances of the
ferromagnets $r_F$, and $\gamma\sim r_F/r_N^*\ll 1$ in the absence of contact
barriers. Efficient spin injection can be ensured by contact barriers. Explicit
formulae for the junction resistance and the spin-valve effect are presented.
