A new model of 3D quantum chaotic billiards is used to investigate glo
bal quantum fluctuations in metallic particles. The quantum system is
described by means of a tight-binding Hamiltonian, in which the energi
es of the orbitals at the surface sites are chosen at random between -
1/2 W and 1/2 W. The results indicate that for values of W of the orde
r of half the bandwidth, the fluctuations of the highest occupied leve
l are already much larger than the corresponding average interlevel sp
acing. This provides a natural explanation for autoionization effects
in granular metals. The model shows diffusive or ballistic behavior de
pending on the value of W, as observed in the energy fluctuations whic
h are larger or smaller than GOE, for the respective behaviors.