  We consider a phase-coherent system of two parallel quantum wires that are
coupled via a tunneling barrier of finite length. The usual perturbative
treatment of tunneling fails in this case, even in the diffusive limit, once
the length L of the coupling region exceeds a characteristic length scale L_t
set by tunneling. Exact solution of the scattering problem posed by the
extended tunneling barrier allows us to compute tunneling conductances as a
function of applied voltage and magnetic field. We take into account charging
effects in the quantum wires due to applied voltages and find that these are
important for 1D-to-1D tunneling transport.
