  An important challenge in the field of topological materials is to carefully
disentangle the electronic transport contribution of the topological surface
states from that of the bulk. For Bi$_2$Te$_3$ topological insulator samples,
bulk single crystals and thin films exposed to air during fabrication processes
are known to be bulk conducting, with the chemical potential in the bulk
conduction band. For Bi$_2$Te$_3$ thin films grown by molecular beam epitaxy,
we combine structural characterization (transmission electron microscopy),
chemical surface analysis as function of time (x-ray photoelectron
spectroscopy) and magnetotransport analysis to understand the low defect
density and record high bulk electron mobility once charge is doped into the
bulk by surface degradation. Carrier densities and electronic mobilities
extracted from the Hall effect and the quantum oscillations are consistent and
reveal a large bulk carrier mobility. Because of the cylindrical shape of the
bulk Fermi surface, the angle dependence of the bulk magnetoresistance
oscillations is two-dimensional in nature.
