  Nearly a decade after the discovery of topological insulators (TIs), the
important task of identifying and characterizing their topological surface
states through electrical transport experiments remains incomplete. The
interpretation of these experiments is made difficult by the presence of
residual bulk carriers and their coupling to surface states, which is not yet
well understood. In this work, we present the first evidence for the existence
and control of bulk-surface coupling in Bi2Te2Se nanoribbons, which are
promising platforms for future TI-based devices. Our magnetoresistance
measurements reveal that the number of coherent channels contributing to
quantum interference in the nanoribbons changes abruptly when the film
thickness exceeds the bulk phase relaxation length. We interpret this
observation as an evidence for bulk-mediated coupling between metallic states
located on opposite surfaces. This hypothesis is supported by additional
magnetoresistance measurements conducted under a set of gate voltages and in a
parallel magnetic field, the latter of which alters the intersurface coupling
in a controllable way.
