  In three-dimensional topological insulators (3D TI) nanowires, transport
occurs via gapless surface states where the spin is fixed perpendicular to the
momentum[1-6]. Carriers encircling the surface thus acquire a \pi Berry phase,
which is predicted to open up a gap in the lowest-energy 1D surface subband.
Inserting a magnetic flux ({\Phi}) of h/2e through the nanowire should cancel
the Berry phase and restore the gapless 1D mode[7-8]. However, this signature
has been missing in transport experiments reported to date[9-11]. Here, we
report measurements of mechanically-exfoliated 3D TI nanowires which exhibit
Aharonov-Bohm oscillations consistent with topological surface transport. The
use of low-doped, quasi-ballistic devices allows us to observe a minimum
conductance at {\Phi} = 0 and a maximum conductance reaching e^2/h at {\Phi} =
h/2e near the lowest subband (i.e. the Dirac point), as well as the carrier
density dependence of the transport.
