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In situ resistance measurements of epitaxial Fe layers and Au/Fe bilayers were used to quantify the scattering in giant magnetoresistance (GMR) spin valve structures. The semiclassical Boltzmann transport equation, incorporating first-principles local density functional calculations, fitted the thickness dependence of the conductivity. Fits to the data indicate that Fe has a large spin asymmetry with bulk relaxation times τ↓=3.0 × 10−14 s and τ↑=2.5 × 10−15 s. These give a conductivity equal to that of bulk Fe. The interface scattering from the Fe/GaAs, the Fe/vacuum, and the Au/vacuum interfaces is purely diffuse. This is in contrast to the high electron reflection coefficients determined from kinematical calculations using scanning tunneling microscope images. Fits to conductivity measurements of Au/Fe/GaAs(001) indicate that the Au films have the conductivity of bulk material modified only by interface scattering. The GMR of Au/Fe/Au/Fe/GaAs(001) structures is 1.8% at room temperature and 2.9% at 10 K. The magnetoresistance is reduced by the presence of partial diffuse scattering at the inner interfaces, as indicated by the fits to both the GMR and the Au conductivity. The GMR in Fe/Au structures is intrinsically low due to a large electron band mismatch between Au and Fe band structures.