  We calculate supercurrent across a two-dimensional topological insulator
subjected to an external magnetic field. When the edge states of a narrow
two-dimensional topological insulator are hybridized, an external magnetic
field can close the hybridization gap, thus driving a quantum phase transition
from insulator to semimetal states of the topological insulator. We find a sign
reversal of the supercurrent at the quantum phase transition revealing
intrinsic properties of topological insulators via Josephson effect.
