Is the Planckian scattering rate universal, and is it a bound
In plain words
Data in several materials suggest the electron scattering rate equals temperature times a fixed combination of Boltzmann's and Planck's constants; it is unknown whether this is a true limit.
Precise statement
With $\hbar/\tau = \alpha k_B T$ extracted from the T-linear resistivity slope and the independently measured effective mass and carrier density, test whether $\alpha = 1$ within error across strange metals and whether any metal sustains $\alpha >> 1$. An answer is a proof of a bound on the transport scattering rate within a defined class of models, or a material with reliably measured $\alpha$ well above 1.
What would settle it
Quantum-oscillation or specific-heat masses combined with resistivity slopes in many materials, plus a theorem bounding $\hbar/\tau$ in a stated class of models.
Status in the literature
Analyses of several hole-doped cuprates (2019) gave $\alpha \sim 1$; no rigorous bound on the transport rate is known.