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Abstract: Fast ionic transport is a defining feature of many solid electrolytes, yet its microscopic origin is unknown, leaving their use in technological applications empirical. Existing measurements probe either the low-frequency transport response or the high-frequency bound polarization, leaving the intermediate frequency regime relatively unexplored. However, ion transport emerges from polarization in this regime. By systematically varying the mobile ion concentration and performing time-domain terahertz spectroscopy (TDTS) in a prototypical glassy electrolyte, we reveal this mesoscopic frequency regime and identify a crossover from bound-current-dominated conduction to conductivity arising from short-range dispersive ionic transport. Our results show that ionic conduction requires the coexistence of three ingredients: a mechanically flexible host network, a large bound current that defines the ultrafast electrodynamic environment, and sufficient mobile ion density to sustain transport-like motion.