400 Bad request in twitter api with asp.net


I am successful in authorizing my app to twitter api.
So next i am trying to fetch the user profile details where i am ended up getting error Bad request 400. What i founded regarding this error in twitter blog is
–>The request was invalid or cannot be otherwise served. An accompanying error message will explain further. In API v1.1, requests without authentication are considered invalid and will yield this response.

request with out authentication??

After i got oauth_token only i am performing a get operation but why i am getting this error??

   if (!(Request.QueryString["oauth_token"] == null))
             string Oauth_Token = Request.QueryString["oauth_token"].ToString();
             string oauth = Request.QueryString["oauth_verifier"];
             var accessToken = OAuthUtility.GetAccessToken(consumerKey, consumerSecret, Oauth_Token, oauth);
              if (accessToken.TokenSecret.Length > 0)
                string url = "https://api.twitter.com/1.1/users/show.json?screen_name=Ajaykhyathi";
                 HttpWebRequest request = (HttpWebRequest)HttpWebRequest.Create(url);
                  request.UseDefaultCredentials = true;
                  request.Method = "GET";
                  WebResponse response = request.GetResponse();// Here i am getting the error...



Pleasea help me some body…