A nanofluidic cross-slot device is designed and fabricated to investigate the effects of slitlike confinement on the electrophoretic stretching of single DNA molecules. The device is capable of trapping and stretching single DNA molecules at the stagnation point of a homogeneous planar elongational electric field. Different from studies of unconfined DNA, the longest relaxation time in slitlike confinement is extension-dependent, and we find the higher extension relaxation time allows better prediction of the drastic increase of extension with applied strain rate in confinement. The low extension relaxation time is important in polymer rotation and small deviations from equilibrium.