  We demonstrate that the weak antilocalization effect can serve as a
convenient method for detecting decoupled surface transport in topological
insulator thin films. In the regime where a bulk Fermi surface coexists with
the surface states, the low field magnetoconductivity is described well by the
Hikami-Larkin-Nagaoka equation for single component transport of
non-interacting electrons. When the electron density is lowered, the
magnetotransport behavior deviates from the single component description and
strong evidence is found for independent conducting channels at the bottom and
top surfaces. The magnetic-field-dependent part of corrections to conductivity
due to the Zeeman energy is shown to be negligible despite non-negligible
electron-electron interactions.
