Hi, It's not a good idea to use the back button when you are already on the authorize.net payment page. Depending on the browser sometimes you might see an additional item or no item in the shopping cart.
http://www.tipsandtricks-hq.com/forum/topic/back-button-from-paypal-page-brings-me-to-an-error-page
However, There is an alternative option in the eStore settings - "Enable Fancy Redirection On Checkout". This will do the redirection in a different way and when your customers use the back button (while they are still being redirected) they will come back to your site.