  The problem of Andreev reflection between a normal metal and a multiband
superconductor is addressed. The appropriate matching conditions for the wave
function at the interface are established on the basis of an extension of
quantum waveguide theory to these systems. Interference effects between
different bands of the superconductor manifest themselves in the conductance
and the case of FeAs superconductors is specifically considered, in the
framework of a recently proposed effective two-band model, in the sign-reversed
s-wave pairing scenario. Resonant transmission through surface Andreev bound
states is found as well as destructive interference effects that produce zeros
in the conductance at normal incidence. Both these effects occur at nonzero
bias voltage.
