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Almost all spintronic transistors (e.g., spin field-effect transistors, spin bipolar transistors, and spin-enhanced MOSFETs) require high efficiency of spin injection from a ferromagnetic contact into a semiconductor channel for proper operation. In this paper, we calculate the efficiency of spin injection from a realistic nonideal ferromagnetic contact into the semiconductor quantum wire channel of a spintronic transistor, taking into account the presence of an axial magnetic field (caused by either the ferromagnetic contact or external agents) and spin orbit interaction. In our calculations, the temperature is assumed to be low enough that phonon scattering is weak and transport is phase-coherent, although not ballistic because of elastic scattering caused by impurities and defects. We consider a single impurity in the channel and show that the conductance depends strongly on the exact location of this impurity because of quantum mechanical interference effects. This is a nuisance since it exacerbates device variability. The ldquosignrdquo of the impurity potential, i.e., whether it is attractive or repulsive, also influences the channel conductance. Surprisingly, at absolute zero temperature, the spin injection efficiency can reach 100% at certain gate biases, even though the ferromagnetic injector is nonideal. However, this efficiency drops rapidly with increasing temperature.