a, Bulk Landau-level spectrum for a Weyl cone without quantum confinement. The blue line denotes the gapless chiral Landau level. b, Bulk Landau-level spectrum for a Weyl cone with quantum confinement along the z axis. The yellow dots denote the discrete energy levels due to quantization in the finite-thickness quantum well. c–d, Representative band structures of Cd3As2 slabs with thicknesses of about 40 nm (c) and about 60 nm (d). The red lines denote the Fermi arc surface states, which persist for the 40-nm-thick films. e, Energy gap as a function of thickness. The finite-size effect is only important for slabs with thicknesses of less than 10 nm.