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Metastable cosmic strings appear in models of new physics with a two-step symmetry breaking G → H → 1, where π1(H) ≠ 0 and π1(G) = 0. They decay via the monopole-antimonopole pair creation inside. Conventionally, the breaking rate has been estimated by an infinitely thin string approximation, which requires a large hierarchy between the symmetry breaking scales. In this paper, we reexamine it by taking into account the finite sizes of both the cosmic string and the monopole. We obtain a robust lower limit on the tunneling factor e − S B even for regimes the conventional estimate is unreliable. In particular, it is relevant to the cosmic string interpretation of the gravitational wave signals recently reported by pulsar timing array experiments. [ABSTRACT FROM AUTHOR]