  The Quantum Spin Hall insulator is characterized by the presence of gapless
helical edge states where the spin of the charge carriers is locked to their
direction of motion. In order to probe the properties of the edge modes, we
propose a design of a tunable quantum impurity realized by a local gate under
an external magnetic field. Using the integrability of the impurity model, the
conductance is computed for arbitrary interactions, temperatures and voltages,
including the effect of Fermi liquid leads. The result can be used to infer the
strength of interactions from transport experiments
