  The conductance of {\em cis/trans} isomers of stilbene molecules connected to
armchair single wall carbon nanotubes is studied in the Landauer formalism
combined with a density-functional based approach. For a given arrangement of
the electrodes, dramatic differences in the transmission between both isomers
are found. For a given isomer, the conductance can be varied by orders of
magnitude by changing the molecule-electrode relative orientation. Both effects
can be explained by a simple, physically transparent interference rule, which
suggests a straightforward conductance control in such molecular systems by
different switching mechanisms.
