  Study of the dephasing in electronic systems is not only important for
probing the nature of their ground states, but also crucial to harnessing the
quantum coherence for information processing. In contrast to well-studied
conventional metals and semiconductors, it remains unclear which mechanism is
mainly responsible for electron dephasing in three-dimensional (3D) topological
insulators (TIs). Here, we report on using weak antilocalization effect to
measure the dephasing rates in highly tunable (Bi,Sb)$_2$Te$_3$ thin films. As
the transport is varied from a bulk-conducting regime to surface-dominant
transport, the dephasing rate is observed to evolve from a linear temperature
dependence to a sublinear power-law dependence. While the former is consistent
with the Nyquist electron-electron interactions commonly seen in ordinary 2D
systems, the latter leads to enhanced electron dephasing at low temperatures
and is attributed to the coupling between the surface states and the localized
charge puddles in the bulk of 3D TIs.
