What does this equation mean?

A tilted cone crossing 1 on a band diagram is not automatically a horizon an electron experiences. For the crossing to be a physical event rather than a line on a chart, a carrier has to travel across it ballistically — without being scattered by disorder — for at least as long as the horizon takes to act on it. Turning that single requirement into arithmetic sets a minimum mobility: too dirty a crystal, and disorder buries the horizon under ordinary resistive scattering before any carrier gets close enough to feel the tipped cone. At the paper’s design point, a tenth-of-a-kelvin horizon demands a mobility on the order of 7$\times$10^5\,$\text{cm}^2$/$\text{V·s}$ , with carriers needing to cross roughly fourteen micrometres without scattering to register it.