The conformational and dynamical properties of polyelectrolytes depend strongly on their ionic environment. Such effects are even more significant when a polyelectrolyte is confined. In this study we have used λ-DNA confined in a nanoslit as a model polyelectrolyte. We experimentally study the diffusivity and longest relaxation time as a function of ionic strength and confinement. We find that the change in DNA diffusivity and relaxation time can be quantitatively explained by Blob theory with an electrostatically mediated DNA persistence length and effective width. Surprisingly, it is found that instead of the persistence length, the effective width of DNA (which characterizes the electrostatic repulsion between segments) is the main driving force for significant change in DNA dynamics. Our results provide important insight into DNA and other polyelectrolytes behavior under confinement.