  We report on the observation of gate-tunable proximity-induced
superconductivity and multiple Andreev reflections (MAR) in a bulk-insulating
BiSbTeSe$_2$ topological insulator nanoribbon (TINR) Josephson junction (JJ)
with superconducting Nb contacts. We observe a gate-tunable critical current
($I_C$) for gate voltages ($V_g$) above the charge neutrality point
($V_{CNP}$), with $I_C$ as large as 430 nA. We also observe MAR peaks in the
differential conductance ($dI/dV$) versus DC voltage ($V_{dc}$) across the
junction corresponding to sub-harmonic peaks (at $V_{dc} = V_n =
2\Delta_{Nb}/en$, where $\Delta_{Nb}$ is the superconducting gap of the Nb
contacts and $n$ is the sub-harmonic order). The sub-harmonic order, $n$,
exhibits a $V_g$-dependence and reaches $n = 13$ for $V_g = 40$ V, indicating
the high transparency of the Nb contacts to TINR. Our observations pave the way
toward exploring the possibilities of using TINR in topologically protected
devices that may host exotic physics such as Majorana fermions.
