Conduction electrons flow like honey

In most conductors, crystal defects and phonons scatter conduction electrons and impede the flow of current. But graphene exhibits anomalously weak electron–phonon scattering and can also be prepared in such a pure state that, at temperatures of a few degrees kelvin, an electron driven by a voltage difference moves ballistically. That is, the electron travels unimpeded through the bulk of the material until it hits a boundary, deposits some momentum there, and bounces back into the bulk to move freely until it again ricochets off a boundary. At higher temperatures, conduction electrons scatter off each other rather than off phonons. Due to those interactions, the electrons collectively flow as a viscous liquid, a phenomenon called electron hydrodynamics.