Isomer depletion serves as a prerequisite for a variety of important applications. Several mechanisms are proposed to trigger the isomer depletion. In the previous work [Chiara et al., Nature (London) 554, 216 (2018).], isomer depletion with an exceptionally high probability in ${}^{93m}\mathrm{Mo}$ was reported and attributed to nuclear excitation by electron capture (NEEC). However, contradictory probability values were subsequently reported from both experimental and theoretical aspects. This Letter reports a new and more precise measurement of the isomer depletion of high-energy ${}^{93m}\mathrm{Mo}$ ions during the slowing-down processes in lead and carbon foils via a low-background beam-based experimental approach. By employing a purified isomer beam, the depletion probabilities are determined to be $2.0(2)\times {10}^{-5}$ in lead and $4.7(13)\times {10}^{-6}$ in carbon, agreeing well with the inelastic nuclear scattering calculations. We conclude that the observed isomer depletion in ${}^{93m}\mathrm{Mo}$ arises from inelastic nuclear scattering rather than the previously proposed NEEC mechanism.

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