We study theoretically the conductance of heterostructures with ferromagnetic (F) conductors and a two-dimensional electron gas with Rashba (R) spin-orbit interaction using a two-channel Landauer formula. Assuming a one-dimensional model, we first find the S matrix for the FR interface. This result is then applied to different devices such as a FRF structure, first suggested by Datta and Das [Appl. Phys. Lett. 56, 665 (1990)]. We find analytic results for the conductance for the case of collinear magnetization.