  We analyze the spin expectation values for injected spin-polarized electrons
(spin vectors) in a [001]-grown Rashba-Dresselhaus two-dimensional electron gas
(2DEG). We generalize the calculation for point spin injection in semi-infinite
2DEGs to finite-size spin injection in bounded 2DEGs. Using the obtained spin
vector formula, significance of the channel direction for the Datta-Das
transistor is illustrated. Numerical results indicate that the influence due to
the finite-size injection is moderate, while the channel boundary reflection
may bring unexpected changes. Both effects are concluded to decrease when the
spin-orbit coupling strength is strong. Hence [110] is a robust channel
direction and is therefore the best candidate for the design of the Datta-Das
transistor.
