  We measure the conductivity of neutral fermions in a cubic optical lattice.
Using in-situ fluorescence microscopy, we observe the alternating current
resultant from a single-frequency uniform force applied by displacement of a
weak harmonic trapping potential. In the linear response regime, a
neutral-particle analogue of Ohm's law gives the conductivity as the ratio of
total current to force. For various lattice depths, temperatures, interaction
strengths, and fillings, we measure both real and imaginary conductivity, up to
a frequency sufficient to capture the transport dynamics within the lowest
band. The spectral width of the real conductivity reveals the current
dissipation rate in the lattice, and the integrated spectral weight is related
to thermodynamic properties of the system through a sum rule. The global
conductivity decreases with increased band-averaged effective mass, which at
high temperatures approaches a T-linear regime. Relaxation of current is
observed to require a finite lattice depth, which breaks Galilean invariance
and enables damping through collisions between fermions.
