We present experimental observations at low temperature of subharmonic gap structures in high-quality Nb/${\mathrm{AlO}}_{\mathit{x}}$/Nb tunnel junctions, with relatively low critical current density. Samples having small, though appreciable, differences in the energy gaps show split structures which permit an interpretation in terms of the multiparticle tunneling (MPT). A detailed analysis and discussion of the experimental data in terms of the MPT theories is presented. In particular, the amplitudes and the temperature dependence of the subgap structures and the voltage dependence of the multiparticle current are investigated. Finally, a model that considers the tunnel barrier as consisting of randomly distributed thicknesses is proposed to overcome the failure of the theoretically predicted structure amplitudes for a uniform barrier. Within this framework, the experimental data of this work and those recently obtained by other groups are discussed.

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