  The in-plane resistivity anisotropy is studied in strain-detwinned single
crystals of FeSe. In contrast to other iron-based superconductors, FeSe does
not develop long-range magnetic order below the nematic/structural transition
at $T_{s}\approx$90~K. This allows for the disentanglement of the contributions
to the resistivity anisotropy due to nematic and magnetic orders. Comparing
direct transport and elastoresistivity measurements, we extract the intrinsic
resistivity anisotropy of strain-free samples. The anisotropy peaks slightly
below $T_{s}$ and decreases to nearly zero on cooling down to the
superconducting transition. This behavior is consistent with a scenario in
which the in-plane resistivity anisotropy in FeSe is dominated by inelastic
scattering by anisotropic spin fluctuations.
