  We study coherent transport in density tunable micro-devices patterned from
thin films of the topological insulator (TI) Bi2Se3. The devices exhibit
pronounced electric field effect, including ambipolar modulation of the
resistance with an on/off ratio of 500%. We show that the weak antilocalization
(WAL) correction to conductance is sensitive to the number of coherently
coupled channels, which in a TI includes the top and bottom surface and the
bulk carriers. These are separated into coherently independent channels by the
application of gate voltage and at elevated temperatures. Our results are
consistent with a model where channel separation is determined by a competition
between the coherence time and surface-bulk scattering time.
