In STEM mode, the electron coherence is defined by
the coherence length seen at the condenser aperture.
Coherence length (Lc) is the propagation distance of a wave from a coherent source to a point where the wave (e.g. an electron wave) maintains a specified degree of coherence. Interference is strong within a coherence length of the source, but not beyond it. The coherence length is inversely proportional to the energy spread of the electron beam. It has a magnitude of the order of hundreds of nm in EMs (electron microscopes).