  The dynamics of Ag(111) surface state electrons confined to nanoscale
hexagonal and triangular vacancy islands are investigated using scanning
tunneling spectroscopy. The lifetimes of quantised states with significant
amplitude near the centers of the vacancies are weakly affected by the geometry
of the confining cavity. A model that includes the dependence of the lifetime
on electron energy, vacancy size, step reflectivity and the phase coherence
length describes the results well. For vacancy islands with areas in the range
$\approx 40$--$220 {\rm nm}^2$ lossy scattering is the dominant
lifetime-limiting process. This result and a corrected analysis of published
experimental data improve the consistency of experimental and calculated
surface state lifetimes.
