https://api.constantcontact.com/ws/customers/ is not a valid URL that would retrieve anything. To access some resources in your account, you would need to use a URL such as:
https://api.constantcontact.com/ws/customers/{user-name)/
https://api.constantcontact.com/ws/customers/{user-name)/contacts
https://api.constantcontact.com/ws/customers/{user-name)/lists
You can find a full list of all the valid URL's for our API available in our REST API Reference Guide.
