We report the observation of quantized conductance in high-mobility three-dimensional Dirac semimetal Cd3As2 nanowire and nanoribbon p−n junctions. By employing suspended device geometries with dual local gates, we form tunable p−n junctions and realize ballistic transport across sub-micron channel lengths. In a wide nanoribbon device with a channel width of ∼330 nm, conductance plateaus appear at integer multiples of 2e2/h in the n−n regime under high magnetic fields. Numerical simulations suggest that these features represent unresolved spin split subbands due to the smaller subband spacing in wider channels and support the interpretation that the observed quantization may originate from surface-state-dominated conduction. In contrast, narrower nanoribbons and nanowires exhibit conductance steps of 1e2/h, demonstrating spin-resolved subbands likely due to enhanced confinement effects. From spin-resolved subband spectroscopy, we extract an effective Landé g-factor of ∼43 for the first subband in the bulk gap, establishing these nanostructures as a prospective platform for fault-tolerant quantum electronics.