  In an incoherent metal, transport is controlled by the collective diffusion
of energy and charge rather than by quasiparticle or momentum relaxation. We
explore the possibility of a universal bound $D \gtrsim \hbar v_F^2/(k_B T)$ on
the underlying diffusion constants in an incoherent metal. Such a bound is
loosely motivated by results from holographic duality, the uncertainty
principle and from measurements of diffusion in strongly interacting
non-metallic systems. Metals close to saturating this bound are shown to have a
linear in temperature resistivity with an underlying dissipative timescale
matching that recently deduced from experimental data on a wide range of
metals. This bound may be responsible for the ubiquitous appearance of high
temperature regimes in metals with $T$-linear resistivity, motivating direct
probes of diffusive processes and measurements of charge susceptibilities.
