  The edge states of two-dimensional time-reversal topological insulators
support a perfect helical conductance on wide ribbons due to the absence of
backscattering. Here, we study the changes in the transport properties of
topological insulator nanoribbons by introducing a constriction along the
ribbon. This setup allows the edge states to hybridize, leading to reflections
at the ends of the constriction. We find that the electronic states running
along one edge can be reflected back along the opposite edge multiple times,
giving rise to Fabry-P\'erot resonant vortices within the constriction with
well-defined conductance peaks. We show that magnetic barriers allow one to
manipulate these peaks and obtain significant changes in the system
spin-resolved magnetoconductance.
