A key challenge in condensed matter research is the optimization of topological insulator (TI) compounds for the study and future application of their unique surface states. Truly insulating bulk states would allow the exploitation of predicted surface state properties, such as protection from backscattering, dissipationless spin-polarized currents, and the emergence of novel particles. Towards this end, major progress was recently made with the introduction of highly resistive Bi$_2$Te$_2$Se, in which surface state conductance and quantum oscillations are observed at low temperatures. Nevertheless, an unresolved and pivotal question remains: while room temperature ARPES studies reveal clear evidence of TI surface states, their observation in transport experiments is limited to low temperatures. A better understanding of this surface state suppression at elevated temperatures is of fundamental interest, and crucial for pushing the boundary of device applications towards room-temperature operation. In this work, we simultaneously measure TI bulk and surface states via temperature dependent optical spectroscopy, in conjunction with transport and ARPES measurements. We find evidence of coherent surface state transport at low temperatures, and propose that phonon mediated coupling between bulk and surface states suppresses surface conductance as temperature rises.
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