Electron transport in Weyl semimetals has attracted significant attention in the condensed matter community due to the solid-state realization of Weyl fermions. While magnetotransport in in pseudospin-1/2 (Weyl) systems is well established, its extension to higher-pseudospin fermions remains a frontier with critical implications for transport phenomena in materials with multifold fermions. We present a rigorous quasiclassical analysis of longitudinal magnetotransport in pseudospin-1 fermions, advancing beyond conventional models that assume constant relaxation times and neglect the orbital magnetic moment and global charge conservation. Our study uncovers a magnetic-field dependence of the longitudinal magnetoconductance: it is positive and quadratic in $B$ for weak internode scattering and transitions to negative values beyond a critical internode scattering strength. Notably, the critical threshold is lower for pseudospin-1 fermions compared to their pseudospin-1/2 counterparts. We show analytically that the zero-field conductivity is affected more strongly by internode scattering for pseudospin-1 fermions than conventional Weyl fermions. Furthermore, we extend our study to fourfold-degenerate fermions (pseudospin 3/2), which exhibit multiple Fermi surfaces, and uncover the nuanced nature of magnetotransport arising from distinct scattering channels. These insights provide a foundational framework for interpreting recent experiments on multifold fermions and offer a road map for studying magnetotransport in candidate materials with space group symmetries 199, 214, and 220.

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• Magnetotransport
• Transport phenomena
• Weyl fermions
• Topological materials
• Weyl semimetal
• Semiclassical methods