  Exact quantitative understanding of plasmon propagation along nanowires is
mandatory for designing and creating functional devices. Here we investigate
plasmon transmission through top-down fabricated monocrystalline gold nanowires
on a glass substrate. We show that the transmission through finite-length
nanowires can be described by Fabry-P\'{e}rot oscillations that beat with
free-space propagating light launched at the incoupling end. Using an extended
Fabry-P\'{e}rot model, experimental and simulated length dependent transmission
signals agree quantitatively with a fully analytical model.
