Macroscopic magnetotransport measurements at Bi films grown epitaxially on Si(111) substrates have been carried out at 10 K. The magnetoconductance curves reveal two characteristic regimes, which are assigned to magnetotransport by surface and bulk states, respectively. In contrast to bulk, backscattering, i.e., weak localization, is strongly restricted for the charge carriers in the spin-polarized surface bands, and a classical magnetoresistance behavior was found. While the surface-state conductivity was found to be as high as 4×10−4Ω−1/□, the bulk conductivity is extremely low, possibly due to quantum confinement of the bulk band structure. © 2011 American Physical Society.