  By measuring the transmission of near-resonant light through an atomic vapor
confined in a nano-cell we demonstrate a mesoscopic optical response arising
from the non-locality induced by the motion of atoms with a phase coherence
length larger than the cell thickness. Whereas conventional dispersion theory
-- where the local atomic response is simply convolved by the Maxwell-Boltzmann
velocity distribution -- is unable to reproduce the measured spectra, a model
including a non-local, size-dependent susceptibility is found to be in
excellent agreement with the measurements. This result improves our
understanding of light-matter interaction in the mesoscopic regime and has
implications for applications where mesoscopic effects may degrade or enhance
the performance of miniaturized atomic sensors.
