  Wavefunctions of a relativistic heavy quark-light quark $(Q,q)$ system
described by a Dirac hamiltonian are analyzed. By assuming that the confinement
potential is a Lorentz scalar (S), the slope of the Isgur-Wise function is
calculated at zero recoil point. The result obtained is $\xi^\prime(1)=-0.93\pm
0.05.$ This means that the solutions are perfectly consistent. If relativistic
corrections in the light quark wave functions are included the result is
$\xi^\prime(1)=-1.01\pm 0.04.$ From heavy-light data this suggest that if
relativistic effects are considered, scalar confinement is reliable in low
orbital angular momentum states.
