We develop a theoretical framework for planar quasiballistic Josephson junctions, where multiple superconducting leads are coupled through a large, nearly ballistic normal metal crystal. Our approach, based on quasiclassical Eilenberger equations, accounts for the dominant role of electron reflections from the crystal surfaces or single impurities, a mechanism distinct from both purely ballistic and diffusive limits. We calculate the critical current between superconducting leads for various geometries, examining its dependence on temperature and magnetic field. Crucially, we demonstrate that in multiterminal setups, the junctions are not independent but form a strongly coupled system. The theory successfully explains key experimental observations from a companion work [Polevoy et al., Phys. Rev. Lett. 137, 166001 (2026)], including a nonmonotonic dependence of the critical current on the interlayer length, providing a foundation for designing and understanding complex multiterminal Josephson systems.

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• Ballistic transport
• Coherence length
• Critical current
• Josephson effect
• Mesoscopics
• Josephson junctions
• Nanostructures
• Single crystal materials
• Superconducting devices
• Thin films
• Green's function methods