Topological insulators are expected to exhibit robust edge-state transport protected by time-reversal symmetry, yet experimental conductance often falls below the quantized limit due to intrinsic and interfacial effects. Here, we demonstrate that the orbital character of topological edge states, governed by spin-orbit coupling and edge termination, plays a critical role in electron injection and transport. Using first-principles calculations combined with nonequilibrium Green's function methods, we investigate PbSe nanoribbons as a model system. We identify an orbital filtering mechanism arising from spin-orbital-entangled edge states, where the $j=1/2$ and $j=3/2$ components of the $p$ orbitals obey symmetry-imposed selection rules at the interface. This effect limits conductance even in the absence of disorder; when present, disorder further promotes an interplay between topological edge states and resonant edge-localized states originating from dangling bonds. Our findings highlight the importance of orbital engineering and angular momentum symmetry mismatch in the design of topological devices and call for an examination of interface physics in quantum materials regarding the topological protection.

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• Ballistic transport
• 2-dimensional systems
• Nanoribbon
• Topological materials
• Density functional theory