  In this Letter we report on lateral diffusion measurements of excitons at low
temperature in double quantum wells of various widths. The structure is
designed so that excitons live up to 30 $\mu$s and diffuse up to 500 $\mu$m.
Particular attention is given to establishing that the transport occurs by
exciton motion. The deduced exciton diffusion coefficients have a very strong
well-width dependence, and obey the same power law as the diffusion coefficient
for electrons.
