The inelastic scattering length (Ls) is a length scale of fundamental importance in condensed matters due to the relationship between inelastic scattering and quantum dephasing. In quantum anomalous Hall (QAH) materials, the mesoscopic length scale Ls plays an instrumental role in determining transport properties. Here we examine Ls in three regimes of the QAH system with distinct transport behaviors: the QAH, quantum critical, and insulating regimes. Although the resistance changes by five orders of magnitude when tuning between these distinct electronic phases, scaling analyses indicate a universal Ls among all regimes. Finally, mesoscopic scaled devices with sizes on the order of Ls were fabricated, enabling the direct detection of the value of Ls in QAH samples. Our results unveil the fundamental length scale that governs the transport behavior of QAH materials
