  Topological insulators (TIs) are new insulating materials with exotic surface
states, where the motion of charge carriers is described by the Dirac equations
and their spins are locked in a perpendicular direction to their momentum.
Recent studies by angle-resolved photoemission spectroscopy have demonstrated
that a conventional two-dimensional electron gas can coexist with the
topological surface state due to the quantum confinement effect. The
coexistence is expected to give rise to exotic transport properties, which,
however, have not been explored so far. Here, we report a magneto-transport
study on single crystals of the topological insulator BiSbTe3. Besides
Shubnikov-de Haas oscillations and weak anti-localization (WAL) from the
topological surface state, we also observed a crossover from the weak
anti-localization to weak localization (WL) with increasing magnetic field,
which is temperature dependent and exhibits two-dimensional features. The
crossover is proposed to be the transport manifestation of the coexistence of
the topological surface state and two-dimensional electron gas on the surface
of TIs.
