We report on magnetotransport measurements in InAs nanowires under a large magnetic field (up to 55 T), providing a spectroscopy of the one-dimensional electronic band structure. Large modulations of the conductance mediated by a control of the Fermi energy reveal the Landau fragmentation, carrying the fingerprints of the confined InAs material. Our numerical simulations of the magnetic band structure consistently support the experimental results and reveal key parameters of the electronic confinement.