  Topological insulators (TI), with characteristic Dirac-fermion topological
surface states (TSS), have emerged as a new class of electronic materials with
rich potentials for both novel physics and device applications. However, a
major challenge with realistic TI materials is to access, distinguish and
manipulate the electronic transport of TSS often obscured by other possible
parallel conduction channels that include the bulk as well as a two-dimensional
electron gas (2DEG) formed near the surface due to bending of the bulk bands.
Such a (Schrodinger-fermion) 2DEG represents topologically-trivial surface
states, whose coexistence with the TSS has been revealed by angle resolved
photoemission spectroscopy. Here we show that simple manipulations of surface
conditions can be used to access and control both types of surface states and
their coexistence in bulk-insulating Bi2Te2Se, whose surface conduction is
prominently manifested in temperature dependent resistance and nonlocal
transport. The trivial 2DEG and TSS can both exhibit clear Shubnikov-de Haas
oscillations in magnetoresistance, with different Berry phases ~0 and ~pi that
distinguish their different topological characters. We also report a deviation
from the typical weak antilocalization behavior, possibly due to high mobility
TSS. Our study enables distinguishing, controlling and harnessing electronic
transport of TI surface carriers with different topological natures.
