  The correlation between particle and wave descriptions of electron-matter
interactions is analyzed by measuring the delocalization of an evanescent field
using electron microscopy. Its spatial extension coincides with the
energy-dependent, self-coherence length of propagating wave packets that obey
the time-dependent Schr\"odinger equation and undergo a Goos-H\"anchen shift.
In the Heisenberg limit they are created by self-interferences during
coherent-inelastic Coulomb interactions with a decoherence phase {\Delta}{\phi}
= 0.5 rad and shrink to particle-like dimensions for energy losses of more than
1000 eV.
