  Three-dimensional topological insulator (3D TI) nanowires display various
interesting magnetotransport properties that can be attributed to their
spin-momentum-locked surface states such as quasiballistic transport and
Aharonov-Bohm oscillations. Here, we focus on the transport properties of a 3D
TI nanowire with a gated section that forms an electronic Fabry-P\'erot (FP)
interferometer that can be tuned to act as a surface-state filter or energy
barrier. By tuning the carrier density and length of the gated section of the
wire, the interference pattern can be controlled and the nanowire can become
fully transparent for certain topological surface-state input modes while
completely filtering out others. We also consider the interplay of FP
interference with an external magnetic field, with which Klein tunneling can be
induced, and transverse asymmetry of the gated section, e.g., due to a
top-gated structure, which displays an interesting analogy with Rashba
nanowires. Due to its rich conductance phenomenology, we propose a 3D TI
nanowire with gated section as an ideal setup for a detailed transport-based
characterization of 3D TI nanowire surface states near the Dirac point, which
could be useful towards realizing 3D TI nanowire-based topological
superconductivity and Majorana bound states.
