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In STEM mode, the electron coherence is defined by the coherence length seen at the condenser aperture.

Coherence length (L_c) 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).