How to make a REST call to Magento 2?

I am trying to get going with Magento 2’s REST API interface, and I did the following:

1. I made a new “Integration”, which gave me the Consumer Key and Secret and the Access Token and Secret.

2. I setup a call like this (using an external REST tool):

   GET https://[url]/rest/[code]/V1/categories
   Content-Type: application/json
   Authorization: Bearer <Consumer Secret>
   
   

But I always get this HTTP400 error:

{
"message": "Specified request cannot be processed.",
"trace": null
}

Which does not give me much info on where I am going wrong.

How can I make REST calls to my Magento 2 setup, using the integration keys?