The changes in the energy spectra of cosmic ray electrons, protons, and &agr; particles above roughly 50 MeV/nucleon, as observed during the period 1966--1972, have all been theoretically reproduced by numerical solutions of the steady state transport equation for cosmic rays in the solar system. These solutions have been obtained by smoothly varying the rigidity and radial dependence in a nonseparable model of the interplanetary diffusion coefficient, which is characterized by four parameters. The diffusion coefficient which is implied by these numerical solutions is then compared with the observed power spectra of interplanetary magnetic field fluctuations, validity of the quasi-linear adiabatic theory being assumed. It appears that the changes occurring in the two observables, i.e., diffusion coefficient and magnetic power spectra, agree qualitatively surprisingly well. Uncertainties and simplifying assumptions implied in the theory and data do not permit comparison of the two observables in absolute magnitude with good accuracy, but it is shown that they are not in conflict. It is concluded that the nonseparable model used for the diffusion coefficient matches, within the limitations of theoretical and observational accuracy, the available data of cosmic ray modulation as well as of interplanetary magnetic field fluctuations.