Get url of thumbnail from the media uploader

I want to select image from the WordPress 3.5 media uploader. I can get the image URL with the following code, but it gets the full size image. I want to get the thumbnail image url,how can I get that?

 var custom_uploader;
 $('.upload-image').click(function(e) {
        e.preventDefault();

        if (custom_uploader) {
            custom_uploader.open();
            return;
        }

        custom_uploader = wp.media.frames.file_frame = wp.media({
            title: 'Choose Image',
            button: {
                text: 'Choose Image'
            },
            multiple: false
        });

        //When a file is selected, grab the URL
        custom_uploader.on('select', function() {
            attachment = custom_uploader.state().get('selection').first().toJSON();
            var abc = attachment.url;    //this is full image url. 
            alert (abc);
        });

        custom_uploader.open(); 
    });

You can debug the result of attachment by :

console.log(attachment); 

and if the thumbnail size available you can retrieve it using :

 var thumb = attachment.sizes.thumbnail.url;
 alert(thumb);