  Accurate knowledge of electron transport properties is vital to understanding
the information provided by liquid argon time projection chambers (LArTPCs).
Ionization electron drift-lifetime, local electric field distortions caused by
positive ion accumulation, and electron diffusion can all significantly impact
the measured signal waveforms. This paper presents a measurement of the
effective longitudinal electron diffusion coefficient, $D_L$, in MicroBooNE at
the nominal electric field strength of 273.9 V/cm. Historically, this
measurement has been made in LArTPC prototype detectors. This represents the
first measurement in a large-scale (85 tonne active volume) LArTPC operating in
a neutrino beam. This is the largest dataset ever used for this measurement.
Using a sample of $\sim$70,000 through-going cosmic ray muon tracks tagged with
MicroBooNE's cosmic ray tagger system, we measure $D_L = 3.74^{+0.28}_{-0.29}$
cm$^2$/s.
