  We present model calculations for the Landauer conductance of tunnel
junctions. The tunneling of free electrons through a rectangular potential
barrier is considered. The conductance of a finite number of barriers was
calculated using a transfer matrix method. The periodic arrangement of the same
barriers was described by a Kronig-Penney model to calculate the band structure
and from that the conductance of a point contact in the ballistic limit. The
comparison of both results elucidated the importance of the boundary
conditions. Caused by resonant scattering in the superlattice the conductance
is overestimated by the order of 1/t, the transmission coefficient of the
single barrier. In the case of metallic multilayers these interferences are of
minor importance. In conclusion, the application of the Landauer formula to
periodic lattices to describe the tunneling conductance of a single barrier is
not correct.
