  We calculate the tunneling conductance \sigma between the surface states on
the opposite sides of the ultra-thin film of a topological insulator in a
parallel magnetic field B_y. The parallel magnetic produces a relative shift of
the in-plane momenta of the two surfaces states. An overlap between the shifted
Fermi circles and their spin structure define an unusual dependence of the
tunneling conductance \sigma(B_y) on the magnetic field. Because the spin of
the electronic surface states in topological insulators is locked with
momentum, the spin-polarization of the tunneling current can be controlled by
magnetic field B_y.
