We present evidence of nontrivial topology in the kagome semimetal $\mathrm{N}{\mathrm{i}}_3\mathrm{I}{\mathrm{n}}_2{\mathrm{S}}_2$ from quantum oscillation experiments at high magnetic fields up to 31 T and theoretical investigations by first-principles calculations. Angle-dependent de Haas–van Alphen oscillations reveal the presence of quasi-two-dimensional hole and three-dimensional electron pockets, consistent with first-principles calculations, along with signatures of magnetic breakdown across these pockets. In particular, we find that the smallest cyclotron orbit, $F_{\alpha }\approx 10$ T, enclosing theoretically predicted endless Dirac nodal lines, has a relatively low effective mass of $0.183m_0$ and a Berry phase close to π. Furthermore, this orbit has a high quantum mobility of 0.88 ${\mathrm{m}}^2{\mathrm{V}}^{-1}{\mathrm{S}}^{-1}$, an order of magnitude larger than that of the heavier charge carriers. Infrared spectroscopy reveals linearly increasing optical conductivity with photon energy above 45 meV. These experimental results, as confirmed by first-principles calculations, support that massive Dirac fermions, formed by a small gap opening due to spin-orbit coupling, possess nontrivial topology and can contribute considerably to electrical transport. We also observe linear magnetoresistance above 12.7 T, which can be attributed to the quantum magnetoresistance of the Dirac fermions concentrated on the zeroth Landau level.

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• Landau levels
• Spin-orbit coupling
• Topological materials
• de Haas-van Alphen effect
• Kagome lattice
• Density functional theory
• Quantum oscillation techniques
• Torque magnetometry