  In the two-dimensional case the transition between ordinary and topological
insulator states can be described by a massive Dirac model with the mass term
changing its sign at the transition point. We theoretically investigate how
such a transition manifests itself in resonant transport via localized helical
edge states. The resonance occurs in the middle of the band gap due to a zero
edge-state mode which is protected by the time-reversal symmetry, also when
coupled to the conducting leads. We obtain the explicit dependence of the
resonant conductance on the mass parameter and an external magnetic field. The
proposal may be of practical use, allowing one to determine the orbital
g-factor of helical edge states in two-dimensional topological insulators.
