  With increasing attention in Majorana physics for possible quantum bit
applications, a large interest has been developed to understand the properties
of the interface between a $s$-type superconductor and a topological insulator.
Up to this point the interface analysis was mainly focused on in situ prepared
Josephson junctions, which consist of two coupled single interfaces or to
ex-situ fabricated single interface devices. In our work we utilize a novel
fabrication process, combining selective area growth and shadow evaporation
which allows the characterization of a single in situ fabricated
Nb/$\mathrm{(Bi_{0.15}Sb_{0.85})_2Te_3}$ nano interface. The resulting high
interface transparency is apparent by a zero bias conductance increase by a
factor of 1.7. Furthermore, we present a comprehensive differential conductance
analysis of our single in situ interface for various magnetic fields,
temperatures and gate voltages. Additionally, density functional theory
calculations of the superconductor/topological insulator interface are
performed in order to explain the peak-like shape of our differential
conductance spectra and the origin of the observed smearing of conductance
features.
