  Quantum physics rules the dynamics of small objects as they interact over
microscopic length scales. Nevertheless, quantum correlations involving
macroscopic distances can be observed between entangled photons as well as in
atomic gases and matter waves at low temperatures. The long-range nature of the
electromagnetic coupling between charged particles and extended objects could
also trigger quantum phenomena over large distances. Here, we reveal a
manifestation of quantum mechanics that involves macroscopic distances and
results in a nearly complete depletion of coherence associated with which-way
free-electron interference produced by electron--radiation coupling in the
presence of a distant extended object. We illustrate this effect by a rigorous
theoretical analysis of a two-path electron beam interacting with a
semi-infinite plate and find the inter-path coherence to vanish proportionally
to the path separation at zero temperature and exponentially at finite
temperature. Besides the fundamental interest of this macroscopic quantum
phenomenon, our results suggest an approach to measuring the vacuum temperature
and nondestructively sensing the presence of distant objects.
