  Strong spin-orbit coupling in topological insulators results in the
ubiquitously observed weak antilocalization feature in their magnetoresistance.
Here we present magnetoresistance measurements in ultra thin films of the
topological insulator Bi_2Se_3, and show that in the 2D quantum limit, in which
the topological insulator bulk becomes quantized, an additional negative
magnetoresistance feature appears. Detailed analysis associates this feature
with weak localization of the quantized bulk channels, providing thus evidence
for this quantization. Examination of the dephasing fields at different
temperatures indicates different scattering mechanism in the bulk vs the
surface states.
